In the wake of the recent and still ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local load consumers has become common practice. As a consequence of the competition between utilities and the emerging need to optimize assets, substantially increased amounts of power are transmitted through the existing networks, invariably causing transmission bottlenecks and oscillations of parts of the power transmission systems.
Interconnected alternating current generators remain in synchronism because of the self-regulating properties of their interconnection. Hence, if a first generator deviates from its synchronous speed, power is transferred from the other generators in the system in such a way that the speed deviation is reduced. Since moments of inertia of the generators also come into play, this results typically in speed over-corrections and the whole system or a part thereof starts swinging in the same manner as a set of interconnected pendulums. In general, these electromagnetic oscillations with a frequency of less than a few Hz are stable and considered acceptable as long as they decay. They are initiated by the normal small changes in the system load, and they are a characteristic of any power system. However, an increase in the transmitted power of a few MWs may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in lost of synchronism, lost of interconnections and ultimately the inability to supply electric power to the customer.
An operator can control the power that the generators should supply under normal operating conditions and automatic control mechanisms are responsible for fast adjustments which are necessary to maintain the system voltages and line frequency (e.g. 50 Hz) within design limits following sudden changes in the system. These controls are necessary for any interconnected power system to supply power of the required quality. However there is no warning to the transmission operator if a new operating condition causes the abovementioned oscillatory modes to become lightly damped and thus potentially dangerous. Appropriate monitoring of the power system can help the operator to accurately assess power system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed damping equipment.
In the article “Estimation of Electro-Mechanical Parameters using Frequency Measurements” by M. Hemmingsson, O. Samuelsson, K. O. H. Pedersen and A. H. Nielsen, IEEE 2001, 0-7803-6672-7 (p. 1172) information about electro-mechanical mode parameters such as oscillation frequency and damping have been extracted from measurements made at a 230V wall-outlet during normal operation of The power system. Batches of measurements of the line frequency were taken, each lasting for a time window of at least 10 min, and subsequently analyzed. Spectral analysis of the instantaneous frequency revealed two well known electromechanical modes oscillating at 0.35 Hz and 0.57 Hz. A pole estimation procedure based on a stationary, time discrete model driven by white noise allowed to approximate the corresponding damping of the two modes. Due to the batch sampling technique, no on-line or real-time analysis of the power system was possible.
Electric power transmission and distribution systems or networks comprise high-voltage tie lines for connecting geographically separated regions, medium-voltage lines, and substations for transforming voltages and for switching connections between lines. For managing the network, time-stamped local information about the network, in particular currents, voltages and load flows, can be provided by newly installed Phasor Measurement Units (PMU), c.f. the article “PMUs—A new approach to power network monitoring”, ABB Review 1/2001, p. 58. A plurality of phasor measurements collected from throughout the network at a central data processor in combination provide a snapshot of the overall electrical state of the power system.